1. Field of the Invention
The present invention relates to an antenna driving device.
2. Description of Related Art
Conventionally, in a smart entry system or a tire air pressure monitoring system for a vehicle, a non-contact automatic ticket examination system, and the like, there is used an antenna driving device in which a transmission antenna (an LC antenna or an RLC antenna) is driven at resonance frequency f(=1/(2π√LC)).
As examples of a conventional technique related to the above description, there are JP-A-2007-216869 and JP-A-2011-120216 (which are respectively referred to as Patent Document 1 and Patent Document 2 in the following description).
Here, in a conventional antenna driving device, one of a sine wave driving method and a pulse wave driving method is usually used as an antenna driving method. However, the sine wave driving method has a problem that power consumption and heat generation are large though harmonic wave noise is hardly generated, while the pulse wave driving method has a problem that harmonic wave noise is easily generated though power consumption and heat generation are small.
Note that Patent Document 1 discloses a vehicle equipment remote control device that can switch between the sine wave driving method and the pulse wave driving method in accordance with an operating situation, but there is room for studying a different method for switching the antenna driving method.
In addition, in the conventional antenna driving device, a radio wave transmission range is determined by amount of driving current flowing in the transmission antenna. Therefore, in order to accurately control the radio wave transmission range, a current feedback control method is effective, in which an output voltage of a power supply circuit (i.e., a supply voltage to an antenna driving circuit) is controlled so that the driving current flowing in the transmission antenna is maintained at a constant value. However, the current feedback control method may cause a restriction in system operation because it needs more time for starting the power supply circuit than a voltage feedback control method in which an output voltage of the power supply circuit is maintained at a constant value.
In addition, in order to secure stability of antenna driving, the conventional antenna driving device continues the output operation of the power supply circuit and the antenna driving circuit not only when the antenna is driven (i.e., the antenna is supplied with power) but also when the antenna is halted (i.e., the antenna is not supplied with power). Therefore, current is consumed not only when the antenna is driven but also when the antenna is halted. However, because it is necessary to operate the smart entry system for a vehicle also when engine is stopped, for example, it is important to reduce current consumption of the antenna driving device mounted in the vehicle as much as possible, so as to reduce consumption of battery energy.